The present invention relates in general to RF modulators used for coupling audio signals to an audio reproduction system via an RF antenna input, and, more specifically, to an RF modulator system having high isolation of the receiving antenna input when the modulator is operating.
RF modulators are used for coupling an audio signal into an audio system via its radio tuner when an auxiliary input to the audio system is not available. In an automotive audio system, for example, it may be desired to install a multimedia accessory such as a DVD player, a video cassette player, or a CD disk changer so that audio signals from the accessory can be heard over the speakers of the vehicle audio system. A head unit for such a system may often lack an available auxiliary input for receiving an accessory audio signal. The modulator modulates the accessory audio signal according to the modulation scheme used by a radio reception band of the radio tuner (e.g., FM) so that an auxiliary input in the audio system becomes unnecessary.
The modulated accessory signal may be coupled to the input of the radio tuner by short-range wireless broadcasting (i.e., through the air) or by a direct hardwired connection to the radio receiver's antenna input. When a hardwired connection is employed, a relay is typically used so that only the antenna or the modulator are connected to the antenna input at one time. The relay separates the radio frequency output of the modulator from the vehicle receiving antenna to help prevent the broadcast of the modulated accessory signal from the antenna when the modulator is operating. Disconnecting the antenna from the antenna input of the radio receiver while the modulator is operating also reduces interference from radio stations broadcasting on the same or nearby frequencies as the modulator.
A typical RF modulator may operate using frequency modulation at the lower end of the FM broadcast band. The frequency may be selectable between several standard FM channels (e.g., 87.9 MHz and 88.1 MHz) to avoid using a channel that is also being used by a local radio station transmitter. Even when a direct connection of the RF modulator to the antenna input is made via a relay, however, it has continued to be desirable or even necessary to use a modulator frequency different from any local broadcast stations because the broadcast signal has continued to be picked up at the antenna input thereby causing interference even without a direct antenna connection.
Due to the size and construction of the mechanical relays or micro-relays used for switching the antenna or the RF modulator to the antenna input of the radio receiver, incomplete isolation between all the terminals of the relay is achieved. Specifically, leakage capacitance and leakage inductance between terminals of the relay allows 1) radio broadcast signals to be coupled from the antenna line to the radio's antenna input thereby causing interference, and 2) modulated accessory signals to be coupled from the modulator to the antenna line thereby causing unintended RF emissions.